Problem 138 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds are water soluble? (a) \(\mathrm{ZnSO}_{4}\) (b) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right](\mathrm{OH})_{2}\) (c) \(\mathrm{ZnS}\) (d) \(\mathrm{Na}_{2} \mathrm{ZnO}_{2}\)

Step-by-Step Solution

Answer

The soluble compounds are \(\mathrm{ZnSO}_4\) and \(\mathrm{Na}_2\mathrm{ZnO}_2\).

Step 1: Solubility Rules for Sulfates

Identify solubility rules for sulfate compounds. Most sulfates (like Zinc Sulfate, \(\mathrm{ZnSO}_4\)) are soluble in water, except for those of barium, calcium, and lead.

Step 2: Solubility of Amine Complexes

Consider the compound \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right](\mathrm{OH})_{2}\). Many zinc complexes are generally soluble due to the formation of stable ions in solutions unless the counter-ion (here \(\mathrm{OH}^-\)) makes it insoluble.

Step 3: Check Solubility for Sulfides

Examine \(\mathrm{ZnS}\). Sulfides of many metals, including zinc, are typically insoluble in water except for those with group 1 elements and ammonium.

Step 4: Solubility of Alkali Metal Compounds

Investigate \(\mathrm{Na}_2\mathrm{ZnO}_2\). Compounds containing alkali metals like sodium (\(\mathrm{Na}^+\)) are generally highly soluble in water.

Step 5: Conclusion

Summarize findings from previous steps: \(\mathrm{ZnSO}_4\) and \(\mathrm{Na}_2\mathrm{ZnO}_2\) are soluble in water, while \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right](\mathrm{OH})_{2}\) and \(\mathrm{ZnS}\) are insoluble.

Key Concepts

Solubility Rules

Solubility rules are guidelines to predict whether a compound will dissolve in water. They are essential for understanding chemical reactions and solution chemistry.
For example, most sulfate compounds, like \( \mathrm{ZnSO}_4 \), are soluble in water. However, there are exceptions. Sulfates of barium, calcium, and lead do not dissolve well in water.
Understanding solubility rules helps chemists and students predict reaction outcomes and processes effectively.

Zinc Compounds

Zinc compounds show diverse solubility behaviors. These differences are dependent on the accompanying ions in the compound.
For instance, zinc sulfate \( \mathrm{ZnSO}_4 \) is soluble due to its sulfate ion. However, zinc sulfide \( \mathrm{ZnS} \) is not because metal sulfides are usually insoluble, except when paired with group 1 elements. Moreover, zinc amine complexes provide variability as some are soluble, while others are not based on the stabilizing ions.
Understanding these nuances aids in predicting the compound's behavior in water.

Sulfates

Sulfates are salts formed from sulfuric acid containing the sulfate ion \( \mathrm{SO}_4^{2-} \). They are highly relevant in various chemical processes.
Generally, sulfates like \( \mathrm{ZnSO}_4 \) dissolve in water, benefiting industries, especially where solubility is crucial for processing.
Exceptions such as barium sulfate provide unique challenges and advantages, like in medical imaging, where its insolubility is a benefit.
Knowing about sulfates and their interactions in water helps in designing and understanding chemical systems.

Amine Complexes

Amine complexes, like \( \left[\mathrm{Zn}\left(\mathrm{NH}_3\right)_4\right](\mathrm{OH})_2 \), consist of metal ions bonded to amines, typically ammonia molecules. These complexes form through coordinate bonds wherein the lone pairs on nitrogen bond to the metal center.
Solubility of these complexes relies heavily on the other ions associated with them. While amine complexes can be stable in water, ions like \( \mathrm{OH}^- \) may render the overall compound insoluble.
Understanding amine complexes aids in applications across pharmaceuticals, where solubility can dictate the drug's effectiveness.